The leaves of the perennial herb, Stevia rebaudiana Bert., accumulate quantities of intensely sweet compounds known as steviol glycosides. Whilst the biological function of these compounds is unclear, they have commercial significance as alternative high potency sweeteners.
These sweet steviol glycosides have functional and sensory properties that appear to be superior to those of many high potency sweeteners. In addition, studies suggest that stevioside can reduce blood glucose levels in Type II diabetics and can reduce blood pressure in mildly hypertensive patients.
Steviol glycosides accumulate in Stevia leaves where they may comprise from 10 to 20% of the leaf dry weight. Stevioside and rebaudioside A are both heat and pH stable and suitable for use in carbonated beverages and many other foods. Stevioside is between 110 and 270 times sweeter than sucrose, rebaudioside A between 150 and 320 times sweeter than sucrose. In addition, rebaudioside D is also a high-potency diterpene glycoside sweetener which accumulates in Stevia leaves. It may be about 200 times sweeter than sucrose. Rebaudioside M is a further high-potency diterpene glycoside sweetener. It is present in trace amounts in certain stevia variety leaves, but has been suggested to have a superior taste profile.
Steviol glycosides have traditionally been extracted from the Stevia plant. In Stevia, (−)-kaurenoic acid, an intermediate in gibberellic acid (GA) biosynthesis, is converted into the tetracyclic dipterepene steviol, which then proceeds through a multi-step glycosylation pathway to form the various steviol glycosides. However, yields may be variable and affected by agriculture and environmental conditions. Also, Stevia cultivation requires substantial land area, a long time prior to harvest, intensive labour and additional costs for the extraction and purification of the glycosides.
More recently, interest has grown in producing steviol glycosides using fermentative processes. WO2013/110673 and WO2015/007748 describe microorganisms that may be used to produce at least the steviol glycosides rebaudioside A and rebaudioside D.
Further improvement of such microoganisms is desirable in order that higher amounts of steviol glycosides may be produced and/or additional or new steviol glycosides and/or higher amounts of specific steviol glycosides and/or mixtures of steviol glycosides having desired ratios of different steviol glycosides.